What makes a killer?

There's a somewhat controversial approach to psychology that says some, if not many, of our behaviors are hardwired into us. According to this science, humans evolved with certain behavioral traits that they passed down to subsequent generations. Over time, these behaviors can change. The approach falls under the umbrella of evolutionary psychology.

There's a lack of consensus about evolutionary psychology even among its proponents. According to David Sloan Wilson, part of the reason there's such controversy around the subject is because of an early popular theory posed by Leda Cosmides and John Tooby [source: Huffington Post]. Wilson states this view of evolutionary psychology is narrow and misleading. Among his complaints with their work include the way he says Cosmides and Tooby limited the environment of evolutionary adaptedness as a series of environments in which people lived over the course of their evolutionary process. As a result, Wilson said, Tooby and Cosmides say the human mind developed many pieces designed to solve particular problems encountered in those environments. Wilson believes Cosmides and Tooby oversimplify elements of the human experience with their theory that there are two basic human natures -- one for men and one for women. In addition, he's also critical of the scope of their theory, saying that they didn't take into account variation across populations and the brain's rapid evolution over a relatively short time. Wilson points out there are many other approaches to explaining human behavior from an evolutionary point of view.

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In their chapter on evolutionary psychology as it relates to homicide in the book "Evolutionary Psychology and Violence," David M. Buss and Joshua D. Dunley hypothesize that aggression is a trait inherent in humans that dates back to the prehistoric era. People had to compete with one another for resources. Sometimes this competition became violent. As a result, humans who survived developed parallel sets of skills. One set helped people survive aggression. The other set helped humans inflict greater harm on competitors.

According to this hypothesis, people became more adept at both avoiding and inflicting harm over generations. From a high-level perspective, you could say that based on this hypothesis we are all capable of killing. Many proponents of evolutionary psychology, including Buss and Dunley, don't deny that other factors play a role in turning someone into a killer. They acknowledge that a person's circumstances will influence him or her as well. But at our core, we are all killers.

Critics of evolutionary psychology say that our minds aren't as hardwired as these psychologists suggest. They point to how quickly the mind has evolved since our prehistoric days -- the brain is much faster than the popular presentation of evolutionary psychology can justify. Critics like David Brooks say that humans are too complicated and adaptive for evolutionary psychology to hold much meaning [source: Brooks].

What's the flip side of the coin? What is it that could condition a person into becoming a killer?